The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the invention relate to a method for the reduction of sound travelling in a wave-guide located in the air above the ground.
Under specific circumstances noise, for example air-craft noise during the first part of take-off when the air-craft is gaining speed at a runway, can be very disturbing at a great distance from the source of the noise. This especially applies for sound frequencies in the range from 15 Hz to 40 Hz. In the above example of aircraft noise an increase of low frequency noise above all occurs when the aircraft still has contact with the runway, and runs its engines at high power, whereas further the wind conditions are such that the wind is blowing substantially towards an area of interest (the area suffering from the disturbing noise) with the right (vertical) wind gradient. Other factors which may be of influence are, among others, the (vertical) temperature gradient and/or a combination of wind and temperature gradients.
Under such conditions a situation may be created in which noise emanating from the source of noise in a specific range of angles will propagate above the ground in a so-called wave-guide between the ground surface and a virtual upper limit. Sound waves will successively reflect downwards at the upper limit due to the (vertical) gradient (the effective speed of sound increases with altitude) in the sound speed caused by the summation of the effects of wind gradient and temperature gradient, and upon reaching the ground surface will reflect upwards again, and so on. Basically the reflection at the ground surface is a result of an acoustic impedance boundary present at the ground.
The above wave-guide effect especially occurs in a range of sound frequencies from about 15 Hz to about 40 Hz. The sound propagating through this wave-guide is less attenuated during propagation in comparison to the attenuation that occurs with spherical propagation of sound.
State of the art attempts to reduce disturbing noise substantially comprise structures such as shields, hills, trees and vegetation located between the source of noise and the area of interest. When applied for reducing noise from aircraft at an airfield, such known structures are not compatible with safety requirements calling for free, unobstructed areas alongside the runway. Moreover such structures only are effective in reducing high frequency noise, whereas noise in the above range of 15 Hz to 40 Hz hardly is reduced.